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Experimental Evolution of Aging, Growth, and Reproduction in Fruitflies

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Experimental Evolution of Aging, Growth, and Reproduction in Fruitflies Experimental evolution of aging, growth, and reproduction in fruitflies S. C. Stearns*, M. Ackermann, M. Doebeli, and M. Kaiser Zoology Institute, University of Basel, Rheinsprung 9, CH-4051 Basel, Switzerland Edited by May B. Berenbaum, University of Illinois at Urbana-Champaign, Urbana, IL, and approved January 24, 2000 (received for review July 12, 1999) We report in this paper an evolutionary experiment on Drosophila flies (19–22). It succeeds because evolution is more rapid than that tested life-history theory and the evolutionary theory of until recently was expected (23, 24). aging. As theory predicts, higher extrinsic mortality rates did lead Previous artificial selection experiments on Drosophila (25– to the evolution of higher intrinsic mortality rates, to shorter 31) suggested that the evolution of lifespan is constrained by lifespans, and to decreased age and size at eclosion; peak fecundity genetic links between early-life traits, such as early fecundity and also shifted earlier in life. These results confirm the key role of larval competitive ability, and late-life mortality and fecundity extrinsic mortality rates in the evolution of growth, maturation, rates. The longest running artificial selection experiment on the reproduction, and aging, and they do so with a selection regime evolution of aging in fruit flies (25) started in 1976. One set of that maintained selection on fertility throughout life while holding lines is only allowed to reproduce early in life, another only late population densities constant. in life; fecundity and survival are the targets of selection, and the evolutionary responses in traits such as intrinsic mortality are life-history evolution ͉ lifespan ͉ age at maturity ͉ body size ͉ Drosophila measured. The late-reproducing lines have evolved a much longer lifespan and lower intrinsic mortality rates than the early-reproducing lines. This experiment has been repeated in n this paper, we report a case study in experimental evolution another laboratory with a design that avoids unintentional Iwith the fruit fly Drosophila melanogaster that is designed to selection and controls larval density (31). In this second case, the test predictions of life-history theory (1–6) and the evolutionary late-reproducing lines have improved survival and decreased theory of aging (7–11). It did confirm those predictions. The early-life fertility, but there has been no improvement in late-life change in the environment that drove phenotypic change was a fertility, and no correlated responses to selection in the preadult difference in extrinsic adult mortality rates. What is different in period have been observed. In the best field test of life-history this experiment is that mortality was applied in a way closely theory, still continuing in Trinidad (18, 24), extrinsic mortality resembling natural conditions rather than by using traditional rates of guppies are manipulated by exposing them to different artificial selection. Treatments differed only in adult mortality predators. Significant genetically based changes in age at matu- applied twice each week, which maintained selection on fertility rity and fecundity in the predicted direction have been observed, throughout life. and changes in lifespan are expected. Evolutionary Theory of Aging and Life Histories. Evolutionary theory Our Experiment: High Versus Low Adult Mortality. Our experiment predicts the impact of a difference in extrinsic mortality on subjected a stock of wild D. melanogaster to two treatments: high adult mortality (HAM) and low adult mortality (LAM). In intrinsic mortality rates (hence, lifespan) and on growth, matu- contrast to previous experiments, we knew age-specific extrinsic ration, body size, and reproduction. When extrinsic mortality mortality precisely, because we imposed it twice weekly. Thus, rates increase, they lower the probability of survival to a given the problem posed by selection to the organisms differed from age and cause the strength of selection to decline faster with age, the problems posed by previous work (25, 31), because the LAM making an increase in intrinsic mortality rates with age ‘‘more flies were contributing offspring to future generations through- affordable’’ or ‘‘less avoidable.’’ From this concept follows a out their lives, not just at the ends of their lives. This selection central prediction of the evolutionary theory of aging (11): regime, which maintains selection on fertility throughout adult Higher extrinsic mortality rates should lead to higher intrinsic life, is a much closer approximation of the kind of selection that mortality rates and a decrease in lifespan, which is a prediction is likely to be found in nature than is selecting for flies that lay adumbrated by Weismann (7) and Medawar (8), explicit in the eggs only very early or very late in life (25, 31). Moreover, work of Williams (9), quantitative in the research of Hamilton because the population density of both the adults (200 per (10) and Charlesworth (1), and consistent with comparative population cage) and the larvae (250 per unit of larval medium) evidence (11–13). was the same in both treatments, the responses can be attributed Extrinsic mortality rates also affect the evolution of other solely to differences in mortality rates with no confounding life-history traits. Higher extrinsic adult mortality rates should effects of larval or adult density. lead to higher reproductive effort early in life and, for age- but We checked that the predictions of both the evolutionary not stage-dependent life histories (5), to more rapid develop- theory of aging and the reproductive-effort model held for our ment and eclosion at an earlier age and a smaller size (1–6). experimental conditions by analyzing computer models simulat- ing the evolutionary dynamics of phenotypes endowed with Experimental Evolution and Artificial Selection. Recently, a new tool has been exploited to test such predictions: experimental evo- This paper was submitted directly (Track II) to the PNAS office. lution (14). In contrast to artificial selection, in which the Abbreviations: HAM, high adult mortality; LAM, low adult mortality. experimenter determines which trait is selected, in experimental *To whom reprint requests should be addressed. E-mail: [email protected]. evolution, the experimenter creates the conditions under which The publication costs of this article were defrayed in part by page charge payment. This a prediction should hold and lets the evolving population article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C. determine with which traits the problem will be solved. This §1734 solely to indicate this fact. EVOLUTION approach has yielded important insights with bacteria (14–16), Article published online before print: Proc. Natl. Acad. Sci. USA, 10.1073͞pnas.060289597. algae (17), poeciliid fish (18), and temperature adaptation in Article and publication date are at www.pnas.org͞cgi͞doi͞10.1073͞pnas.060289597 PNAS ͉ March 28, 2000 ͉ vol. 97 ͉ no. 7 ͉ 3309–3313 different life-history traits and age schedules of intrinsic mor- talities. In the simulations, these phenotypes were subjected to selection regimes with different adult mortalities that corre- sponded to the regimes used in the experiment. The models assumed the standard constraining relations among develop- ment time, body size, and fecundity that are known to Drosophila researchers (5). The prediction that the HAM treatment should evolve higher intrinsic mortalities held for a range of assump- tions about the shapes of intrinsic-mortality curves, including Gompertz (32) and sigmoid. The predictions of the reproductive- effort model also were confirmed. The computer models pre- dicted that HAM flies would evolve shorter development times, smaller size at eclosion, higher fecundity early in life, and lower fecundity later in life than did LAM flies. These expectations have the following intuitive explanation. In this experiment, extrinsic adult mortality starts when flies enter a population cage at the age of 14 days from egg. Under the high mortality in the HAM treatment, most flies can only reproduce for a few days before they are killed; they should evolve high fecundity at the age of 14 days by eclosing early and by being at the peak of their age-fecundity curve when they enter the cage. In the LAM treatment, most flies can reproduce for several weeks; fecundity late in life is important and can be increased by eclosing later at a larger size, which, however, delays maturity and lowers fecundity at age 14 days. Note that if the shift in mortality rates occurs precisely at eclosion, rather than at a particular age, then an increase in adult mortality selects for a longer juvenile period (at low risk) and a larger size with higher fecundity at eclosion. Methods The founder stock of 820 flies consisted of 10 virgin males and 10 virgin females from each of 41 isofemale lines that were collected in and near Basel, held as isofemale lines for several years, then bred together in a single cage for about five gener- Fig. 1. Intrinsic and total mortality in the population cages. Intrinsic mor- ations to yield a genetically variable population. To found the tality is indicated by the irregular lower lines, one line per replicate. Negative mortalities indicate that too many flies accidentally were added in the last selection lines, we collected eggs once a week for 5 weeks to get check. Total mortality (intrinsic plus extrinsic mortality) is given by the thick five age classes and to avoid the cycles of reproduction and age straight line. The symbols indicate when there were not enough flies available classes that persist in populations founded with flies of one age. to achieve the target mortality because not enough eggs had been laid 14 days The experiment started November 15, 1993, is continuing, and previously. ᮀ, Replicate 1; E, replicate 2; ‚, replicate 3. Filled symbols indicate is maintained on a day–night cycle of 12 h of light, 25°C͞12hof females, and open symbols indicate males. (Upper) HAM.
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